The present invention relates to an device for illuminating plants, comprising: a lamp; - a reflector that has a profiled surface facing the lamp for directing the radiation from the lamp towards the plants; an insert that is placed between the lamp and the plants to be illuminated and has a profiled surface, wherein said profiled surface faces the lamp and the profiled surface of the reflector for directing the radiation from the lamp and for directing the radiation reflected by the reflector towards the plants; a fitting that accommodates the insert, the lamp and the reflector. The invention furthermore relates to a method for illuminating plants, wherein the device according to the invention is used as well as an insert clearly intended for use with the device. The device for illuminating plants described above is known. British Patent 74,988 describes such an device, where the electronic ballast and one or more lamps are integrated in a triangular insert that is fixed to a fitting at both ends. With this arrangement the insert influences the path of the radiation from the lamp and the radiation reflected by the reflector and impedes a direct view into the lamp. It is true that with this known device the direction of radiation from the lamp and the reflector can be influenced by means of the insert. However, this influence is permanent, or at least cannot be adjusted during use of the lamp and is predetermined by the shape of the profiled surfaces and the geometry of the interacting reflector and insert. This will be found to be a disadvantage in many fields of application. In horticulture, for example, it is important to be able to illuminate the plants in a flexible manner. This is important, inter alia, in order to be able to adapt to a specific growth stage of the plants or in order to adapt to the distribution of the plants over the greenhouse area. In the state of the art the requisite flexibility is achieved by electronic control of the light intensities of the lamp, the use of lamps with different light intensities or adjustment of the position of the lamp with respect to the plants. However, the direction of the radiation that originates from the lamp is not modified by these measures, as a result of which either a large amount of radiant energy is lost or some portions of the plants are under-illuminated whilst other portions of the plants are over-illuminated. The invention overcomes the abovementioned disadvantages by providing an device mentioned in the preamble that is suitable for illuminating plants in a flexible manner, wherein the insert can be coupled in a detachable manner to the device in order to direct the radiation of the lamp differently in the coupled position to the direction in the uncoupled position. As a result of this measure the direction of the radiation can be adapted to a specific growth stage of the plants or to a different spatial distribution of the plants over the greenhouse area, by simply coupling an insert to the device without the light intensity of the lamp or the position of the lamp with respect to the plants having to be modified. The invention can advantageously be employed with relatively tall fruit-bearing plants where the fruits grow relatively high above the ground surface, such as tomatoes and peppers. In an initial stage of cultivation of such plants it will be desired to concentrate the radiation preferably centrally below the lamp. In a later stage when the plants have grown broader and are bearing fruit, it will be desired to direct the radiation more to the side of the lamp. According to the invention this is thus achieved by coupling an appropriate insert to the illumination device. According to the invention it is important that the insert can be coupled to the device relatively rapidly and easily and can be removed again without damage. This can be achieved, for example, in that the insert can be coupled to the reflector and/or the fitting by a snap-fit connection, hi another preferred embodiment the insert can be coupled to the reflector and/or the fitting by movement of a locking member, such as a slider or a hook. In both embodiments the insert can be coupled or uncoupled rapidly and by hand. hi another preferred embodiment the lamp is elongated and the profiled surface of the insert and of the reflector is profiled in the longitudinal direction of the lamp. The advantage of this embodiment is that the distribution of radiation with respect to an imaginary plane that extends over the longitudinal direction of the lamp and the vertical direction with respect to the plants to be illuminated, respectively, is essentially symmetrical. As a result the plants can thus be illuminated symmetrically on either side of the lamp. If the plants are arranged in rows, the plants can be essentially uniformly illuminated in the longitudinal direction of the lamp by providing several lamps above the plants with an appropriate spacing. La a special preferred embodiment that can be used with the embodiment from the previous paragraph the profiled surface of the insert has a V profile, the top of which faces the lamp. The advantage of this embodiment is that the radiation from the lamp is directed more towards the side when the insert is in the coupled position. With this arrangement it is advantageous according to the invention if the profiled surface of the reflector has a W profile, the openings of which face the lamp. With this embodiment it is known that by this means the radiation from the lamp is already directed sideways without an insert, the region immediately below the lamp receiving relatively less light. If an insert is coupled in this embodiment, the radiation from the lamp will be directed sideways to an even greater extent. In a preferred embodiment of the invention the distance between the insert and the reflector in the vertical direction with respect to the plants to be illuminated is adjustable. The advantage of this embodiment is that the direction of radiation cannot only be modified relatively broadly by (uncoupling the insert but can also be modified more accurately, specifically by adjusting this distance. In another preferred embodiment the device comprises a multiplicity of fittings, each of which is provided with at least one lamp, a reflector and an insert that can be coupled to the device, hi horticulture the plants are usually arranged in rows in a greenhouse, several lamps preferably being provided above the plants in order to be able to illuminate the complete row. hi a special preferred embodiment the device has displacement means that move the fittings with respect to the plants. If the fittings are moved linearly back and forth in the longitudinal direction with respect to a row of plants, the number of lamps can advantageously be restricted by this means, hi the case of vertical movement of the fittings with respect to the plants the intensity of radiation with which the plants are illuminated is modified, for example adapted to the growth stage of the plants. The invention also relates to an insert that is intended for use with one of the abovementioned embodiments of the device according to the invention. The invention furthermore also relates to a method for the illumination of plants, wherein in a first growth phase the plants are illuminated using an device according to the invention without coupled inserts, with a direction of radiation suited to the first growth phase, and in a second, subsequent growth phase the plants are illuminated using the device with one or more coupled inserts, with a direction of radiation suited to the second growth phase. The advantage of this method is that the illumination of the plants can be adapted quickly and easily to a specific growth phase of the plants by coupling an insert depending on a specific size that has been reached by the plants. In a preferred embodiment of the method the device is moved relative to the plants during part of the first and/or the second growth phase. The advantage of this embodiment is that it provides a supplementary option for adapting the illumination of the plants to a specific growth phase, for example by moving the device in the vertical direction with respect to the plants. The invention will now be explained in more detail on the basis of a description of a preferred embodiment of the invention with reference to the appended drawings, in which: Figure 1 shows, diagrammatically and in cross-section, an device without insert above plants in a first growth phase; Figure 2 shows, diagrammatically and in cross-section, an device with a coupled insert above plants in a second growth phase. It can be seen from Figs 1 and 2 that in a preferred embodiment the device 1 comprises a lamp 2, a reflector 3 and an insert 5. The reflector 3 has a profiled surface 4 facing the lamp and the insert 5 has a profiled surface 6 facing the lamp and the profiled surface of the reflector. The insert 5 can be coupled to the device 1 quickly and easily, for example by a snap- fit connection or by movement of a locking member, such as a slider or a hook. The associated coupling parts are not shown but can consist of components generally available commercially. The insert 5, the lamp 2 and the reflector 3 can be accommodated in a fitting (not shown), which preferably also contains the electronics for controlling the lamp. The insert and the reflector, or in any event at least their profiled surfaces 4, 6, are preferably essentially made of aluminium. These surfaces can optionally be provided with a special coating that promotes the reflection of the radiation and/or optionally influences the spectrum of the radiation. The lamp is preferably a high-pressure gas discharge lamp that is widely used in horticulture under glass and generally has an elongated shape. The device is suspended above plants 7, 8, where the plants 7 in Fig. 1 are in a first growth phase and the plants 8 in Fig. 2 are in a second, later growth phase where fruits 11 have developed. The plants are growing in substrate 9, such as stages or cultivation pots, which are separated by a path 10. The device according to the invention is preferably used with relatively tall fruit-bearing plants, such as tomatoes and peppers. Other applications, such as, for example, additional illumination of plants that are further away from the path 10, are, however, also conceivable. In both figures the (distribution of) the direction of radiation of the lamp is indicated by light and dark areas. If the two figures are compared it can clearly be seen that with a coupled insert 5 the radiation is directed more towards the side than without an insert. Preferably, the profiled surface of reflector 4 has a W profile, the openings of which face the lamp 2. It is known that with such a profile the radiation is already directed somewhat towards the side, at least compared with reflectors with a continuous parabolic curvature, as a result of which less radiation is directed towards the path 10 and more radiation is directed towards the plants 7, 8. Incidentally, this effect is not indicated in Figures 1 and 2. The profiled surface of the insert 6 preferably has a V profile, the top of which faces the lamp 2. This is a very effective shape for directing the radiation more towards the side with a coupled insert. The profiled surface of both the insert 5 and of the reflector 3 are profiled in the longitudinal direction of the elongated lamp so that the plants are illuminated symmetrically on either side of the lamp. According to the invention the distance between the insert 5 and the reflector 3 in a vertical direction with respect to the plants 7, 8 to be illuminated is adjustable. This provides the option of accurately setting the modification in the direction of radiation, or at least more accurately than the modification as a consequence of (uncoupling. A possible displacement is indicated here by the double-headed arrow A, as a result of which the intensity with which the plants are illuminated can be adapted to a specific growth phase of the plants. Another possibility is to move the device in a direction perpendicular to the plane of the drawing. By this means fewer lamps can suffice to illuminate plants placed in rows. With the method according to the invention the plants 7 in the first growth phase are illuminated using an device 1 without insert 5 and after a certain amount of growth the plants 8 are then illuminated using an device with a coupled insert. With this approach the device can preferably be moved with respect to the plants, for example can be moved in one of the abovementioned directions. According to the invention a multiplicity of fittings are provided in order to be able to illuminate larger cultivation areas with the aid of a lighting system. In this context it is advantageous if the fittings can be moved relative to the plants using displacement means. The displacement means (not shown) are means known per se and can comprise guide rails, runners, electric motors, etc.